feat(web): expose the agent-preset roster over the API

`agentPreset.list` gives a browser the deployment's roster so it can offer a
choice when starting a session. Each row carries the id, its `trust`, and
whether it is the current default.

`trust` is on the wire deliberately: a `user` preset is exactly as privileged
as the plugins it names, so a surface that offers one alongside a shipped
preset can say which is which rather than presenting both as vetted.

The domain is read-only. A preset is a composition on disk, so authoring one
is a filesystem act rather than an RPC; and a deployment composing no presets
answers with an empty roster rather than an error, because sharing the host
composition is a valid deployment.

The RPC map made every registration site a type error, so the route, the
response-schema table, the service delegate, and the browser fixture are all
wired rather than only the ones I remembered.

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -23,6 +23,7 @@ function scriptedApi(overrides: {
   host?: Partial<ApiProxy['host']>
   commands?: Partial<ApiProxy['commands']>
   skills?: Partial<ApiProxy['skills']>
+  agentPresets?: Partial<ApiProxy['agentPresets']>
   events?: Partial<ApiProxy['events']>
   goals?: Partial<ApiProxy['goals']>
   settings?: Partial<ApiProxy['settings']>
@@ -86,6 +87,7 @@ function scriptedApi(overrides: {
       ...overrides.commands,
     },
     skills: { list: r => ok(r, { skills: [] }), ...overrides.skills },
+    agentPresets: { list: r => ok(r, { presets: [] }), ...overrides.agentPresets },
     goals: {
       create: err,
       edit: err,